Beam current measuring device and apparatus using the same

ABSTRACT

A beam current measuring device (BMD) capable of measuring beam current while radiating the beam on a target, such as a semiconductor wafer. The BMD at least includes: (a) a detecting section for detecting or collecting a magnetic field corresponding to the beam current; and (b) measuring section including (i) a SQUID sensitive to magnetic flux, and (ii) a feedback coil for carrying feedback current to cancel out a change in the magnetic flux penetrating through the SQUID. With finite beam current except zero penetrating through the detecting section, the SQUID is locked. A BMD of the present invention can be incorporated and used in an ion-implantation apparatus, electron beam exposure apparatus, accelerator, and the like.

FIELD OF THE INVENTION

The present invention relates to a beam current measuring device for accurately measuring a value of ion beam current without interrupting the beam and also relates to an apparatus using the beam current measuring device.

BACKGROUND OF THE INVENTION

Method of accurately measuring a value of ion beam current without interrupting the beam is disclosed in the following reference: “A cryogenic current-measuring device with nano-ampere resolution at the storage ring TARN II, Nuclear Instruments and Methods in Physics Research A 427 (1999) 455-464” (hereinafter referred to as reference 1).

This method determines a beam current value by using a superconducting quantum interference device (hereinafter referred to as a SQUID), i.e. a highly-sensitive magnetic field sensor, and measuring the magnetic filed generated by the beam current.

The beam current measuring device for use in the method is mainly made of: (a) detecting section for detecting a magnetic field corresponding to beam current; (b) a magnetic flux transfer section for transferring magnetic flux to a measuring section; (c) the measuring section including (i) a superconducting element sensitive to the transferred magnetic flux and (ii) a feedback coil for carrying feedback current to cancel out a change in the magnetic flux penetrating through the superconducting element; and (d) a magnetic shielding section made of a superconductor for magnetically shielding the detecting section, the magnetic flux transfer section, and the measuring section from the external space including a space carrying ion beams.

The detecting section is made of a coil in which superconducting wires are wound around a core made of a soft magnetic material. The core of a soft magnetic material collects the magnetic field generated by beam current to induce superconducting current through the coil. The induced superconducting current is transferred to a coil placed adjacent to a SQUID. A change in the superconducting current flowing through the coil in response to a change in beam current tries to change the amount of magnetic flux penetrating through the SQUID. However, the device is structured so that the feedback coil carries feedback current not to change the amount of magnetic flux penetrating through the SQUID and to cancel out the change. Further, because the feedback current is proportional to the change in beam current value, measurement of the feedback current can determine the amount of the change in the beam current value.

Recently, a method of measuring a beam current value using a high-temperature superconductor has been disclosed in “HTS Flux Concentrator For Non-Invasive Sensing Of Charged Particle Beams, ISEC 2001 in page 469-470”. ISEC 2001 stands for 8^(th) International Superconductive Electronics Conference, Jun. 19-22, 2001 Osaka, Japan.

This method uses a cylinder coated with a high-temperature superconductor on the surface thereof as a detecting section. However, on the outer peripheral surface of the cylinder, a bridge part partially made of a high-temperature superconductor is provided. Beam current penetrating through the center of the cylinder induces surface shielding current on the surface of the cylinder. The surface shielding current is concentrated on the bridge part. In this measuring method, the magnetic flux generated by the concentrated surface shielding current is measured by a SQUID.

Experiments are conducted to determine the sectional area of scattered molecular ions by placing such a beam current measuring device in a beam storage ring and measuring attenuation of the beam current value when a circulating ion beam collides with a target gas, deviates from the orbit, and the number of ions decreases.

For example, it is measured how a beam current of several hundred nanoamperes at incident into the storage ring attenuates to several nanoamperes for several dozens of seconds.

According to reference 1, a good linearity of the output of the beam current measuring device is kept up to 2.5 μA. This is a measuring range sufficient to measure beams fluctuating from several hundred nanoamperes to several nanoamperes. Specifically, the SQUID is locked at zero beam current, and thereafter, attenuation of the beams incident into the storage ring is measured. In other words, the amount of a change from zero is measured with respect of zero beam current.

However, the conventional methods have a problem of a narrow measuring range. When the measuring range is widened, measuring accuracy decreases. In other words, the conventional methods cannot measure beam current of several microamperes or larger. For example, in an ion-implantation apparatus for use in semiconductor production, a semiconductor wafer is irradiated with an ion beam ranging from a microampere to several dozens of milliamperes. To control the dose, beam current values must be measured with an error of 1% or smaller.

Application of the conventional beam current measuring device incorporating a SQUID for this use poses a problem. Because the linearity of the output is kept only at several microamperes or smaller, the device cannot be used. Further, when the range in which the linearity of the output is kept is widened from a microampere to several dozens of milliamperes, the sensitivity of the output with respect to the beam current has to be decreased. As a result, there is a problem of a decrease in measuring accuracy.

SUMMARY OF THE INVENTION

The present invention provides a beam current measuring device including: (a) a detecting section for detecting or collecting a magnetic field corresponding to beam current; and (b) a superconducting quantum interference device (SQUID) responsive to magnetic flux. With finite beam current except zero penetrating through the detecting section, the SQUID is locked.

Further the present invention provides an ion-implantation apparatus, an electron beam exposure apparatus and an accelerator comprising the beam current measuring device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a circuit used for examining a performance of a beam current measuring device of the present invention.

FIG. 2 is a graph showing output of simulated current of triangular waves at a frequency of 1 Hz.

FIG. 3 shows a waveform of a power source for current simulating fluctuations of beam current of an ion-implantation apparatus.

FIG. 4 is a graph showing output of simulated current ranging from 15 to 15.24 μA.

FIG. 5 is a drawing for explaining operation of measuring the simulated current ranging from 15 to 15.24 μA.

FIG. 6 is a drawing for showing a beam current measuring device having a detecting section consisted of a high permeability core and a superconducting coil, and a beam location according to the present invention.

FIG. 7 is a drawing for showing a beam current measuring device having a detecting section consisted of a cylinder coated with a high-temperature superconductor on the surface thereof, and a beam location according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In a beam current measuring device of the present invention, beam current is determined in the following manner.

First, a SQUID is locked when the beam current takes a finite value except zero. In other words, with respect of a finite beam current value, the amount of a change from that state is measured. For example, in an ion-implantation apparatus, after a beam for ion implantation is set to substantially a fixed value, the SQUID is locked. Then, fluctuations of the beam current thereafter are measured with the beam current measuring device. The value of the current when the SQUID is locked is measured with a Faraday cup provided on the beam line. Thereafter, the Faraday cup may be moved to a position in which the beam is not interrupted. Alternatively, after the SQUID is locked, the beam can be radiated with a target, e.g. a semiconductor wafer, inserted.

The beam current value is determined by adding the measurement obtained with the Faraday cup when the SQUID is locked and the measurement obtained with the beam current measuring device.

In this manner, the present invention allows measurement of beam current values, which could not be measured with conventional devices because the values are out of the range in which the linearity of output is kept. Then, substantially fixed but slightly fluctuating beams, such as those used for ion implantation in semiconductor production, can accurately be measured. As a result, while a target, e.g. a semiconductor wafer, is irradiated with a beam, the beam current can be measured at the same time.

An example of an embodiment of the present invention is described with reference to the accompanying drawings.

Drawings are schematic and thus do not show accurate dimensional positions.

In this exemplary embodiment, instead of ion beams, a one turn coil carrying simulated current is used for experiments. As a power source of the simulated current, Lecroy's waveform generator LW420 is used. As an oscilloscope, Tektronix's TDS520D is used.

Exemplary Embodiment

FIG. 1 is a block diagram of a circuit used for examining the performance of a beam current measuring device of the present invention. Simulated current flows through an electric wire set in the vicinity of a path corresponding the axis of a beam path of the beam current measuring device. The output of the beam current measuring device 10 is measured on the side of the 2-channel of the oscilloscope 16. The output is equivalent to the potential difference across the feedback resistor 13 of FIG. 1. This potential difference is proportional to a change in ion beam current. The feedback resistance is set to 4.7 kΩ. As necessary, an image of the oscilloscope 16 is shot with a digital camera.

Voltages are converted into current through a resistor of 100 kΩ to provide the simulated current. For example, when a voltage of 10 mV is supplied from a power source 15, a simulated current of 100 nA flows. The voltages generated from the power source 15 are monitored on the side of the 1-channel of the oscilloscope 16. By changing the settings of the power source 15, simulated current of triangular waves, or waves simulating the fluctuations of the beam current of an ion-implantation apparatus is provided. Next, examples of the measurement are given.

FIG. 2 plots the output with respect to simulated current of triangular waves at a frequency of 1 Hz. In this graph, excellent linearity is obtained within the range of approximately ±1.3 μA. The current sensitivity is 6.8 mV/nA.

Further, even when the position of the electric wire set along the path corresponding the axis of the beam path is changed, the similar effects are obtained. Thus, it has been proved that the output does not depend on the position of the current.

FIG. 3 shows a waveform of a power source for current simulating fluctuations of the beam current of an ion-implantation apparatus. Current that is zero at the beginning rises to 15 μA at a certain point. The rise time is for one microsecond. Thereafter, the current fluctuates within the range of 15 μA and 15.24 μA, for approximately 25 seconds. The fluctuations are repeated in a cycle of 25 seconds. The fluctuation width is 0.24 μA, i.e. fluctuations of 1.6% with respect to 15 μA. The output of such simulated current is measured.

After the current rises to 15 μA, the SQUID 11 is locked. Then, the fluctuations of the current are measured. FIG. 4 shows the results of measured output. This graph well reproduces how the current fluctuates. In comparison with a fluctuation width of the simulated current of 0.24 μA, the fluctuation width of the output is 0.247 μA. In other words, the error is 7 nA. This means that a simulated current of 15 μA can be measured with an error of 0.05%. Additionally, the time delay of the output fluctuations is not seen.

Now, with reference to FIG. 5, a description is provided of the reason why simulated current ranging from 15 to 15.24 μA can be measured even though the linearity of output is kept only within the range of +1.3 μA.

At the beginning, the simulated current is zero. At this time, SQUID 11 is not operated. Then, after a current of 15 μA has begun to flow, SQUID 11 is locked. Thus, the range of +1.3 μA can be measured mainly around the value taken when the SQUID 11 is locked. In other words, the range of 15±1.3 μA can be measured. This corresponds to the range of 15 μA±8.7%.

This method is based on the characteristics that SQUID 11 in the sensor section of this device can measure a change in magnetic flux occurring after SQUID 11 is locked.

In this embodiment, a simulated current of 15 μA is measured. However, if the device is designed to decrease or increase its sensitivity according to the fluctuation width of a beam to be used, the device can accommodate to a wide range of current values.

Also when the detecting section is a cylinder coated with a high-temperature superconductor on the surface thereof as shown in FIG. 7, the present invention can be implemented and provide the similar effects. The detecting section 20 of FIG. 7 is a cylinder coated with a high-temperature superconductor on the surface thereof. Area 28 is made of a high-temperature superconductor. Area 29 is made of a metal or an insulator. As shown in the drawing, on the outer peripheral surface, a bridge part partially made of a high-temperature superconductor is provided. Beam current penetrating through the center of the cylinder induces surface shielding current on the surface of the cylinder. The surface shielding current is concentrated on the bridge part. The magnetic flux generated by the concentrated surface shielding current is measured with SQUID 11.

A description is provided hereinafter with reference to FIGS. 6 and 7. A Faraday cup 17 is provided at the after the detecting section 20. At the beginning, the Faraday cup is provided on the track of a beam in a position interrupting the beam. After the beam is drawn from an ion source, the value of the beam current is measured with the Faraday cup 17. Next, SQUID 11 is locked. The value of the beam current taken when SQUID 11 is locked is measured with the Faraday cup 17. The amount of a change in the beam current after SQUID 11 is locked is measured with SQUID 11. Thereafter, the Faraday cup 17 is moved out of the track of the beam, and a target of various kinds of products, such as a semiconductor wafer, is irradiated with the beam. Alternatively, a Faraday cup 17 can be provided at the before the detecting section. In this case, at the beginning, Faraday cup 17 is placed on the track of the beam in a position interrupting the beam. After the beam is drawn from an ion source, the value of the beam current is measured with the Faraday cup 17. Next, the Faraday cup 17 is moved out of the track of the beam, and target 27 of various kinds of products, such as a semiconductor wafer, is irradiated with the beam. Immediately after the irradiation, SQUID 11 is locked.

Measuring section 22 includes SQUID 11, feedback coil 25 for carrying feedback current to cancel out a change in magnetic flux penetrating through SQUID 11.

Control circuit 14 controls measuring section 22 and locks SQUID 11.

Magnetic shielding section 24 includes a superconductor for magnetically shielding the detecting section 20, the magnetic flux transfer section 23, and the measuring section 22 from an external space including a space carrying an ion beam Cryostat 21 is an apparatus used to provide low-temperature environments in which operations may be carried out under controlled conditions.

As described above, a beam current measuring device 10 of the present invention has a wide range of applications when it is desired to accurately measure a substantially fixed but slightly fluctuating beam. For example, a beam current measuring device of the present invention can be incorporated and used in an ion-implantation apparatus, electron beam exposure apparatus, accelerator, and the like. 

1. A beam current measuring device comprising: (a) a detecting section for detecting a magnetic field corresponding to beam current; (b) a superconducting quantum interference device (SQUID); and (c) a Faraday cup; wherein, with finite beam current except zero penetrating through the detecting section, the SQUID is locked.
 2. The beam current measuring device of claim 1, wherein a beam current value taken when the SQUID is locked is measured with the Faraday cup, an amount of a change in the beam current value after the SQUID is locked is measured with the beam current measuring device, and the two values are added.
 3. An ion-implantation apparatus comprising: a beam current measuring device comprising: (a) a detecting section for detecting a magnetic field corresponding to beam current; (b) measuring section including: (i) a superconducting quantum interference device (SQUID); and (ii) a feedback coil for carrying feedback current to cancel out a change in magnetic flux penetrating through the SQUID; (c) a magnetic flux transfer section for transferring the magnetic flux detected by the detecting section to the measuring section; (d) a magnetic shielding section including a superconductor for magnetically shielding the detecting section, the magnetic flux transfer section, and the measuring section from an external space including a space carrying an ion beam; and (e) a Faraday cup; wherein, a beam current value taken when the SQUID is locked with finite beam current except zero penetrating through the detecting section is measured with the Faraday cup, an amount of a change in the beam current value after the SQUID is locked is measured with the beam current measuring device, and the two values are added.
 4. An electron beam exposure apparatus comprising: a beam current measuring device comprising: (a) a detecting section for detecting a magnetic field corresponding to beam current; (b) measuring section including: (i) a superconducting quantum interference device (SQUID); and (ii) a feedback coil for carrying feedback current to cancel out a change in magnetic flux penetrating through the SQUID; (c) a magnetic flux transfer section for transferring the magnetic flux detected by the detecting section to the measuring section; (d) a magnetic shielding section including a superconductor for magnetically shielding the detecting section, the magnetic flux transfer section, and the measuring section from an external space including a space carrying an ion beam; and (e) a Faraday cup; wherein, a beam current value taken when the SQUID is locked with finite beam current except zero penetrating through the detecting section is measured with the Faraday cup, an amount of a change in the beam current value after the SQUID is locked is measured with the beam current measuring device, and the two values are added.
 5. An accelerator comprising: a beam current measuring device comprising: (a) a detecting section for detecting a magnetic field corresponding to beam current; (b) measuring section including: (i) a superconducting quantum interference device (SQUID); and (ii) a feedback coil for carrying feedback current to cancel out a change in magnetic flux penetrating through the SQUID; (c) a magnetic flux transfer section for transferring the magnetic flux detected by the detecting section to the measuring section; (d) a magnetic shielding section including a superconductor for magnetically shielding the detecting section, the magnetic flux transfer section, and the measuring section from an external space including a space carrying an ion beam; and (e) a Faraday cup; wherein, a beam current value taken when the SQUID is locked with finite beam current except zero penetrating through the detecting section is measured with the Faraday cup, an amount of a change in the beam current value after the SQUID is locked is measured with the beam current measuring device, and the two values are added. 